The use of solid state light sources, comprising e.g. light emitting diodes (LED), for illumination and other applications increases rapidly. Solid state lighting holds the promise of providing very efficient illumination devices and technological progress is aimed to further improve the efficacy of the light emitting devices used in such illumination devices.
Often, the illumination devices for such applications are required to provide white light. One approach to obtain such illumination devices is to convert at least a part of the primary light of the solid state light sources to secondary light using phosphor species. White light may be obtained by partial conversion of a blue light with a wavelength converting material comprising such a phosphor. The blue light, emitted e.g. by a light emitting diode (LED) is partially absorbed by the phosphor, causing the phosphor to emit light of a different color, e.g. a yellow light. The blue light emitted by the LED is mixed with the yellow light emitted by the phosphor, and the viewer perceives the resulting mixture of the blue and yellow light as a white light.
The phosphor species, however, emit the secondary light (e.g. the yellow light) with an isotropic light distribution pattern (i.e. the secondary light is emitted over a solid angle of 4 pi). Therefore, at least a part of the secondary light is emitted back in the direction of the primary light source and may be absorbed, thereby reducing the efficacy of the light emitting device. The absorption of the secondary light may be as high as 50%. US 2009/0001399 discloses a method for increasing the luminous efficacy of a white light emitting diode using a LED die and a phosphor. Primary light is emitted by the LED die and converted to secondary light by the phosphor. At least one additional layer or material is provided between the LED die and the phosphor that is transparent for the primary light from the LED die and reflective for the secondary light of the phosphor. The additional layer or material may reduce the absorption of the secondary light.
The transparency and the reflectivity of the additional layer or material for the primary light and secondary light, respectively, are not optimal. Therefore, the efficacy of the light emitting device can still be further improved.